For decades, scientists have been stumped by the signals plants send themselves to initiate photosynthesis, the process of turning sunlight into sugars. UC Riverside researchers have now decoded those previously opaque signals.

For half a century botanists have known that the command center of a plant cell, the nucleus, sends instructions to other parts of the cell, compelling them to move forward with photosynthesis. These instructions come in the form of proteins, and without them, plants won't turn green or grow.

"Our challenge was that the nucleus encodes hundreds of proteins containing building blocks for the smaller organelles. Determining which ones are the signal to them to trigger photosynthesis was like finding needles in a haystack," said UCR botany professor Meng Chen.

The process the scientists in Chen's laboratory used to find four of these proteins is now documented in a Nature Communications paper.

Previously, Chen's team demonstrated that certain proteins in plant nuclei are activated by light, kicking off photosynthesis. These four newly identified proteins are part of that reaction, sending a signal that transforms small organs into chloroplasts, which generate growth-fueling sugars.

Chen compares the whole photosynthesis process to a symphony.

"The conductors of the symphony are proteins in the nucleus called photoreceptors that respond to light. We showed in this paper that both red and blue light-sensitive photoreceptors initiate the symphony. They activate genes that encode the building blocks of photosynthesis."

The unique situation, in this case, is that the symphony is performed in two "rooms" in the cell, by both local (nucleus) and remote musicians. As such, the conductors (photoreceptors), who are present only in the nucleus, must send the remotely located musicians some messages over distance. This last step is controlled by the four newly discovered proteins that travel from the nucleus to the chloroplasts.

This work was funded by the National Institutes of Health, in the hopes that it will help with a cure for cancer. This hope is based on similarities between chloroplasts in plant cells and mitochondria in human cells. Both organelles generate fuel for growth, and both harbor genetic material.

Currently, a lot of research describes communication from organelles back to the nucleus. If something is wrong with the organelles, they'll send signals to the nucleus "headquarters." Much less is known about the activity-regulating signals sent from the nucleus to the organelles.

"The nucleus may control the expression of mitochondrial and chloroplast genes in a similar fashion," said Chen. "So, the principles we learn from the nucleus-to-chloroplast communication pathway might further our understanding of how the nucleus regulates mitochondrial genes, and their dysfunction in cancer," Chen said.

The significance of understanding how photosynthesis is controlled has applications beyond disease research. Human settlements on another planet would likely require indoor farming and creating a light scheme to increase yields in that environment. Even more immediately, climate change is posing challenges for crop growers on this planet.

"The reason we can survive on this planet is because organisms like plants can do photosynthesis. Without them there are no animals, including humans," Chen said. "A full understanding of and ability to manipulate plant growth is vital for food security."

Research Report:Anterograde signaling controls plastid transcription via sigma factors separately from nuclear photosynthesis genes

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